Cytochrome P450s encompass a superfamily of oxidases responsible for the oxidation of numerous endobiotics and thousands of xenobiotics. In addition, in plants, cytochrome P450s play important roles in wound healing, pest resistance, signaling, and anti-microbial and anti-fungal activity.
Capsidiol is a bicyclic, dihydroxylated sesquiterpene produced by many Solanaceous species in response to a variety of environmental stimuli, including exposure to UV (Back et al., Plant Cell. Physiol. 389:899-904, 1998) and infection by microorganisms (Molot et al., Physiol. Plant Pathol. 379-389, 1981; Stolle et al., Phytopathology 78:1193-1197, 1988; Keller et al., Planta. 205:467-476, 1998). It is the primary antibiotic or phytoalexin produced in tobacco in response to fungal elicitation, and it is derived from the isoprenoid pathway via its hydrocarbon precursor, 5-epi-aristolochene (FIG. 1). Several of the biosynthetic enzymes leading up to 5-epi-aristolochene formation have been studied (Chappell, Annu. Rev. Plant Physiol. Plant Mol. Biol. 46:521-547, 1995), especially 5-epi-aristolochene synthase (EAS) (Vögeli and Chappell, Plant Physiol. 88:1291-1296, 1988; Back and Chappell, Proc. Natl. Acad. Sci. U.S.A. 93:6841-6845, 1996; Mathis et al., Biochemistry 36:8340-8348, 1997; Starks et al., Science 277:1815-1820, 1997). EAS commits carbon to sesquiterpene metabolism by catalyzing the cyclization of farnesyl diphosphate (FPP) to 5-epi-aristolochene. However, until the present invention, the enzyme(s) responsible for the conversion of 5-epi-aristolochene to capsidiol has yet to be fully identified and characterized.
Biochemical evidence from previous studies in tobacco (Whitehead et al., Phytochemistry 28:775-779, 1989) and green pepper (Hoshino et al., Phytochemistry 38:609-613, 1995) have suggested that the oxidation of 5-epi-aristolochene to capsidiol occurs in a two step, process with one of the hydroxylation steps being constitutive and the other being mediated by an elicitor-inducible cytochrome P450 (FIG. 1). Because 1-deoxycapsidiol had been isolated from natural sources (Watson et al., Biochem. Soc. Trans. 11:589, 1983), Whitehead et al. (Phytochemistry 28:775-779, 1989), surmised that perhaps the biosynthesis of this intermediate was due to pathogen induction of a corresponding hydroxylase. They therefore prepared synthetic 1-deoxycapsidiol and reported a modest conversion of this compound to capsidiol when fed to control or unelicited tobacco cell cultures. This was further supported by their observation that radiolabeled 5-epi-aristolochene was only converted to capsidiol when fed to elicitor-induced cell cultures but not control cultures. Whitehead et al. (Phytochemistry 28:775-779, 1989) therefore concluded that the 3-hydroxylase, responsible for hydroxylation of 5-epi-aristolochene at C3 to generate 1-deoxycapsidiol, was pathogen/elicitor inducible, while the 1-hydroxylase, responsible for hydroxylating 1-deoxycapsidiol at the C1 to generate capsidiol, was constitutive. Hoshino et al. (Phytochemistry 38:609-613, 1995) added to the observations of Whitehead et al. (Phytochemistry 28:775-779, 1989) by directly measuring 3-hydroxylase-activity in microsomal preparations of arachidonic acid-elicited Capsicum annuum fruits and seedlings. These assays consisted of incubating 5-epi-aristolochene with microsome preparations and subsequently determining the amount of 1-deoxycapsidiol generated by a combination of thin-layer chromatography (TLC) separations and gas chromatography (GC). Their evidence demonstrated that the conversion of 5-epi-aristolochene to 1-deoxycapsidiol was dependent on both NADPH and O2, and that 1-deoxycapsidiol accumulation in vitro was arrested by the P450 antagonists carbon monoxide (Omura and Sato, J. Biol. Chem. 239:2370-2378, 1964), ancymidol (Coolbaugh et al., Plant Physiol. 62:571-576, 1978), and ketoconazole (Rademacher, Annu. Rev. Plant Physiol. Plant Mol. Biol. 51:501-531, 2000).
Recent results suggest that the hydroxylation of 5-epi-aristolochene is an important regulated step in capsidiol biosynthesis. In studies to evaluate the effectiveness of methyl-jasmonate as an inducer of capsidiol biosynthesis in tobacco cell cultures, Mandujano-Chávez et al. (Arch. Biochem. Biophys. 381:285-294, 2000), reported that the modest accumulation of this phytoalexin was accompanied by a strong induction of EAS. This result implied that steps before or after the sesquiterpene cyclase reaction were limiting. Using an in vivo assay measuring the conversion rate of radiolabeled 5-epi-aristolochene to capsidiol, a very limited induction of the hydroxylase activities was observed in cells treated with methyl jasmonate relative to that in fungal elicitor-treated cells. This result pointed to the hydroxylase reactions as a potentially limiting step in capsidiol biosynthesis.